Prior to making a reading or recording, a vibration measuring instrument must ideally wait for its one or more sensors and electronics to ‘settle’. This is required to ensure that any reading made is accurate and is not substantially affected by transient responses of the sensors and electronics. For instance, when a sensor is powered up, its internal pre-amplifier requires time to stabilise. Also, where integrators are used to convert acceleration vibration data into velocity vibration data, or to convert velocity vibration data into displacement vibration data, the integrators require some time to establish a mean level of their input signal to avoid excessive drift in their output.
There may also be conditions resulting from an operator's interaction with the instrument that require a settling time before vibration data is collected. For instance, an operator may have changed the selection of analogue signal conditioning prior to analogue-to-digital conversion, such as changing AC-coupling to DC-coupling. Alternatively or additionally, the conversion sampling rate may have been changed. Even though the sampling rates are typically accurately calibrated, the integration process commonly carried out in vibration instruments provides significant amplification to any slight step change in output level. Further, the instrument's sensors may have been moved recently, which can have a dramatic effect on the integration process.
The amount of time required for settling depends on many factors, including the time the sensors themselves require to power up and stabilise, the number of integration processes required (e.g. two, when a displacement reading is required from an accelerometer), the duration of the recording, which is governed by the maximum frequency of interest or Fmax, the number of spectral lines and the number of averages, and the minimum frequency of interest or Fmin.
Traditionally, to determine the settling time, a table of settling times would be created based on several of the parameters noted above. When a recording was desired, the appropriate amount of settling time was calculated from the table. This process is labour-intensive and time-consuming. Further, to ensure that sufficient settling time was allowed for in all situations, the values in the table were designed to be reasonably conservative. This meant that, in some situations, longer than necessary times were allowed for settling.
It is an object of the present invention to either provide a method and apparatus that reduce the time it takes to collect vibration data by automatically determining the required settling time, or at least provide the public with a useful choice.